enable dynarrays of strings

[charles-cooper]

What I did

as title.

How I did it

the codegen should all work - we just blocked it semantically to match
(static) arrays of strings.

How to verify it

Commit message

Description for the changelog

Cute Animal Picture

[fubuloubu]

Add some test cases

[lgtm-com]

This pull request introduces 1 alert when merging 651100b into 4e38ec4 - view on LGTM.com

new alerts:

• 1 for Unused import

[codecov-commenter]

Codecov Report

Merging #2922 (4b0bacf) into master (4e38ec4) will decrease coverage by 2.27%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2922      +/-   ##
==========================================
- Coverage   88.47%   86.20%   -2.28%     
==========================================
  Files          97       97              
  Lines       10613    11537     +924     
  Branches     2527     2936     +409     
==========================================
+ Hits         9390     9945     +555     
- Misses        770     1095     +325     
- Partials      453      497      +44     

Impacted Files	Coverage Δ
vyper/semantics/types/indexable/sequence.py	87.85% <ø> (-0.26%)	⬇️
vyper/semantics/validation/utils.py	81.04% <ø> (-10.26%)	⬇️
vyper/codegen/function_definitions/utils.py	80.00% <0.00%> (-20.00%)	⬇️
vyper/semantics/types/user/enum.py	67.81% <0.00%> (-12.89%)	⬇️
vyper/builtin_functions/functions.py	85.04% <0.00%> (-9.09%)	⬇️
vyper/codegen/core.py	78.50% <0.00%> (-6.08%)	⬇️
vyper/codegen/expr.py	79.34% <0.00%> (-4.38%)	⬇️
vyper/codegen/types/types.py	85.34% <0.00%> (-3.01%)	⬇️
vyper/ast/nodes.py	91.31% <0.00%> (-2.24%)	⬇️
... and 2 more

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